Process control valves are used in myriad industrial applications for controlling the flow of a fluid. For example, in chemical processing plants or oil refineries, control valves are used to manipulate a flowing fluid to compensate for increases or decreases in demand, or other load disturbances, and thus keep the fluid flow regulated.
The control valve is typically provided as part of a control valve assembly having a control valve, a control valve actuator, and a positioner. The control valve includes an inlet and an outlet, with a movable valve plug therebetween. By adjusting the position of the valve plug, fluid flow through the valve is adjusted. The control valve actuator typically includes a stem connected to the valve plug and provides the necessary motive force for moving the valve plug. The positioner provides a closed loop feedback system that provides pressurized fluid to the control valve actuator (which may include two chambers separated by a diaphragm) and the positioner monitors the position of the valve plug, or more commonly the position of the actuator stem, and compares the position to a desired set point. If a deviation between the actual position and the desired set point is detected which will result in an undesirable flow through the valve, the positioner sends a fluid signal to the actuator to adjust the position of the metering device accordingly.
Such control valve assemblies can be provided in a variety of forms. In one common arrangement the control valve is referred to as a sliding stem valve. In such a valve, a sliding plug or piston is positioned between an inlet and outlet of the valve. Depending on the position of the plug, the cross-sectional area of opening between the inlet and outlet is adjusted, thus allowing more or less fluid to traverse through the valve. The actuator for such a control valve can also be provided in a variety of forms, but commonly has a movable stem directly coupled to a valve stem extending from the valve plug, and employs air pressure, or other fluid pressure, within a diaphragm casing of the actuator to thus cause the actuator stem to move.
The positioner of such a system is typically mounted on the outside of the actuator, and includes a sensor therein adapted to receive a signal from a transmitter mounted on the sliding stem of the actuator. The positioner may include tubing fluidly connecting the positioner to the control valve actuator in order to send fluid signals as control fluid pressure to the actuator. The tubing may be external to the control valve, or the tubing may be incorporated into a housing.
Recently, so called “smart” positioners have been introduced that can diagnose and report certain malfunctions within the control valve. One example of a smart positioner is the FIELDVUE® positioner manufactured by Fisher Controls. Another such positioner is described in U.S. Patent Publication No. 2001/0037159, which is hereby incorporated by reference herein. Such positioners diagnose and report a variety of control valve malfunctions by sending signals to a centralized process controller. Smart positioners may communicate with the centralized process controller via any number of protocols including, but not limited to the FOUNDATION® Fieldbus protocol and the HART™ protocol. While known smart positioners are capable of detecting certain general malfunctions in a control valve, these positioners are not capable of distinguishing between certain types of control fluid leaks within the control valve assembly.
Control fluid leaks in the control valve assemblies can occur between the positioner and the actuator, in the tubing, or in the diaphragm itself. While control fluid leaks in the tubing are generally easy and inexpensive to repair, control fluid leaks in the diaphragm itself are more costly and time consuming to repair. Given that many control valves are located in remote locations in process control systems, repair technicians must carry all parts and tools necessary to repair either a tube leak or a diaphragm leak when a smart positioner reports a control fluid leak.
While the disclosure is susceptible to various modifications and alternative constructions, certain illustrative embodiments thereof have been shown in the drawings and will be described below in detail. It should be understood, however, that there is no intention to limit the disclosure to the specific forms disclosed, but on the contrary, the intention is to cover all modifications, alternative constructions, and equivalents falling within the spirit and scope of the invention.